This invention relates to lasers, and in particular to the use of a visible laser beam to aim an infrared CO.sub.2 laser beam.
Since lasers have found applications in a wide variety of fields, the methods for precisely and conveniently delivering laser energy have been of great importance. Many laser applications such as welding and surgery often require the delivery of a high power infrared laser beam, e.g., a CO.sub.2 laser beam. Because an infrared laser beam is not visible, however, it is difficult to determine where the beam is being directed. Consequently, many delivery systems employ an additional visible laser whose beam is coaxially superimposed on the infrared beam, thus providing a method for aiming.
This combination of infrared and visible lasers, however, has several difficulties which originate in the fact that the conventional method for delivering laser beams employs refractive lenses to focus and guide the laser beams.
The first problem is imprecise aiming. Because of the frequency difference between visible and infrared light, chromatic aberration causes the lenses of the delivery system to refract visible and infrared light rays at slightly different angles. This divergence causes the two beams to be focused at different locations upon delivery. In precision applications, however, it is crucial that the aiming beam and the infrared beam are focused at the same location.
The second problem with the conventional method for delivering both visible and infrared laser light is the necessity to use a red aiming beam due to the inherent limitations in optical materials. Because a single system of lenses must guide both the visible and infrared beams, the lenses must be composed of materials that transmit the light from both the visible and infrared CO.sub.2 lasers. Since most materials have limited transmission ranges, this places significant restrictions on the types of materials which may be used for lenses. For example, although glass and quartz transmit visible light, they are both opaque to the 10.6 .mu.m light from a CO.sub.2 laser (see FIG. 3).
There are materials which do transmit both visible light and infrared CO.sub.2 laser light, but many of them are either toxic or unstable and are therefore not suitable for practical applications. Although there are a few suitable materials which transmit both visible light and infrared CO.sub.2 laser light, the visible light which they do transmit is restricted to the red end of the spectrum. For example, the most suitable material, zinc selenide, has a transmission range from 600 nm up to 18 .mu.m, which includes the 10.6 .mu.m light from a CO.sub.2 laser but only the orange and red portion of the visible spectrum from 600 nm to 770 nm (see FIG. 3). Consequently, all aiming beams for infrared laser delivery are presently restricted to the red end of the spectrum above 600 nm.
Red light, however, is an ineffective color for an aiming beam. In surgical applications, for example, one usually delivers the laser beam to a red target such as the liver or bloody tissue. In this situation a red aiming beam is difficult to see against the red background. Moreover, the human eye is not as sensitive to red light as it is to other colors, making a red aiming beam inherently difficult to see. In short, the red aiming beams presently required for infrared laser delivery are imprecise and difficult to see.